Many printing systems include a plurality of arrays of marking elements, such that the different arrays print different types of dots on a recording medium in order to form an image. A familiar example is a color inkjet printer. The different arrays of marking elements in such a case would be the different groups of nozzles for printing cyan, magenta, yellow and black dots to form the image. (In addition to inkjet nozzles, other types of marking elements include light emitters such as LED's for electrophotography, heaters for thermal imaging, electrodes for electrography, magnetic elements for magnetography, etc.) Different arrays of marking elements can also consist of a first group of marking elements that print dots of a first size and a second group of marking elements that print dots of a second size, or a first group of marking elements that print dots of a color at a relatively high saturation and a second group of marking elements that print dots of substantially the same color but at a relatively low saturation. The dots on the recording medium need to be properly registered with each other or the image quality will be degraded.
The arrays of marking elements in a printer can be provided on a single printhead or on a plurality of discrete printheads. Especially for the case of marking element arrays being disposed on separate printheads, special measures are typically needed to correct for misalignment of different arrays of marking elements, because the mechanical tolerances of alignment of the different printheads may not be adequate to provide proper registration of the dots on the recording medium. In fact, even for different arrays of marking elements made on the same printhead, manufacturing defects or operational conditions can cause the dots from one array to be misaligned relative to the dots from another array.
In a carriage printer, the printhead or printheads are mounted on a carriage that is moved past the recording medium in a carriage scan direction as the marking elements are actuated to make a swath of dots. At the end of the swath, the carriage is stopped, printing is temporarily halted and the recording medium is advanced. Then another swath is printed, so that the image is formed swath by swath. In a carriage printer, the marking element arrays are typically disposed along an array direction that is substantially parallel to the media advance direction, and substantially perpendicular to the carriage scan direction. Corresponding marking elements from the different arrays arrive proximate a given pixel location on the recording medium at different times, so that some types of misalignment can be compensated for by suitable relative timing of actuation of the marking elements. Other types of misalignment can be compensated for by selecting which marking element from one array should correspond to which marking element from a different array for printing the same pixel locations. For example, for ideal registration of the marking element arrays, marking element 1 of cyan would correspond to marking element 1 of yellow, etc. However, for a misregistered set of arrays such that the cyan, magenta and yellow arrays are misaligned relative to the media advance direction, a better choice for improved image quality, for example, might be to have marking element 1 of yellow correspond to marking element 2 of cyan and to marking element 3 of magenta.
In order to know how to compensate appropriately for misalignment of arrays of marking elements, one must measure the misalignment. This is typically done by printing and scanning an alignment test pattern, where the scanning may be done by a document scanner, or by a light emitter and photosensor that are mounted on the carriage, for example.
U.S. Pat. No. 5,448,269 and U.S. Pat. No. 6,478,401 provide examples of printhead alignment test patterns. However, as printhead resolution and image quality increase, there is a need for alignment test methods and registration test patterns having improved accuracy. In addition, some prior art alignment test methods and registration test patterns are susceptible to error due to random dot placement errors (such as from misdirected jets for an inkjet printhead). Therefore there is also a need for reduced sensitivity to image noise in alignment test methods and registration test patterns.